Summer primary productivity across a trophic gradient in Lake Erie and the implications of shifting nutrient drivers on the base of the food web

Abstract

The present study examined primary productivity in Lake Erie during July 2019 and August 2021 using radioisotope (¹⁴Carbon) tracers and compared these values to rates from the 1970s and 1990s. Primary productivity was on average significantly higher in the west basin (59.5 mg C m^-3 h^-1) compared to central (17.1 mg C m^-3 h^-1) and east basins (7.6 mg C m^-3 h^-1) during this time frame characterizing a trophic gradient that ranges from eutrophic to oligotrophic. Larger net plankton (>20 µm) were most productive in the west basin and corresponded with blooms of Cyanobacteria and Bacillariophyceae (diatoms).  Smaller picoplankton (<2 µm) had similar rates of production across the lake and are likely an important vector for energy transfer in all three basins. Because of its large size the central basin accounts for more than half of the lake-wide primary productivity. Despite concerted efforts to reduce phytoplankton biomass through phosphorus abatement, current rates of carbon uptake are at levels similar to those observed in 1970 in the west and central basins and are most likely the consequence of a shifting nutrient regime that includes a higher proportion of soluble reactive phosphorus and increasing nitrogen concentrations. More than 50 years have passed since the original baseline assessment of primary productivity in Lake Erie was conducted in 1970. In that time frame, the status of the lake has shifted from ‘being in an advanced state of eutrophication’ to ‘recovering from eutrophication’ to ‘re-eutrophication’ as it continues to be impacted by anthropogenic stressors; more frequent and consistent measurements of primary productivity are called for in order to improve our understanding of food web dynamics and nutrient management.